Teen Daughter Now Arrested in Alleged Family Plot to Murder Dad

A fourth arrest has been made in the grisly murder of Steven Gibson, who was found stabbed to death on March 1 in his Peoria, Ariz., home in a plan allegedly hatched by his own wife and son, police say.

"She was not present during the murder itself, but our detectives determined that she had knowledge of the plan," police spokeswoman Amanda Jacinto tells PEOPLE.

"We're not discussing the family's possible motive due to the ongoing investigation," Jacinto adds. "More suspects could be charged."

Gibson's stepdaughter, Amber McGonagill, 23, who did not live in the family home, previously told PEOPLE: "My dad was a loving man and was not abusive in any way. He gave my mom anything she would ever ask for, and the two younger children were spoiled to pieces. This is a complete shock to the rest of the family."

On Feb. 28, Gibson's wife Michelle, 41, a nursing school student, allegedly offered $1,000 each to two of her son Steven Jr.'s friends, telling them "they needed to kill [her husband] before he killed her or [his son]," according to court documents filed by prosecutors.

Later that evening, one of the two boys, Erik McBee, 16, who worked at Popeye's Chicken, allegedly beat a sleeping Steven Gibson in the head with a baseball bat in the family home, according to the prosecutors' complaint.

Steven Jr. then later "drove [a] large knife he was holding into his father's chest at least three times," the documents say. "[McBee] then took the knife and slit the victim's throat."

Michelle Gibson, Steven Jr., and McBee were all previously charged with first-degree murder and have so far pleaded not guilty.